Fully differential operational amplifiers have dual inputs and outputs. They typically include a differential input stage which feeds signal currents into a transimpedance stage, which may be considered an output stage. At higher frequencies, about the audible range, such amplifiers become increasingly subject to the transmission of common mode inputs. A common mode input is a signal applied equally and in phase to both of the inputs. Such an input may arise, for example, from noise in one or both supply voltages. Amplifiers which are implemented with field-effect transistors, such as in complementary metal-oxide-semiconductor (CMOS) technology, are particularly susceptible to the effects of common mode inputs at higher frequencies because parasitic capacitances of the transistors provide coupling of extraneous noise to the signal path.
Common mode input suppression of a CMOS amplifier is typically achieved by using a differential input stage. Common mode output suppression may be achieved by the provision of a feedback control feature. In "High-Frequency CMOS Switched-Capacitor Filters for Communications Application" by T. C. Choi et al. in IEEE Journal of Solid State Circuits, vol. SC-18, No. 6, December 1983, pp. 652-664 there is described a folded cascode fully differential CMOS amplifier which includes in the transimpedance stage a pair of common-mode suppression feedback transistors. These transistors have common drains and are each in series with the bias current path of a signal branch adjacent the negative supply voltage node. They operate in the triode mode and have their gates coupled to the respective outputs of the negative and positive signal branches. By responding to the common mode component of the outputs to regulate the bias current accordingly, the feedback transistor pair significantly reduces the common mode component of the output. In doing so, however, it introduces second order harmonic components of the differential mode output signal.